On a number of occasions since the 2019 migration, my SDK3 projects will randomly lose the VRC URL Input Field component, which cannot be fixed by re-importing the SDK3 unity package. Specifically, either completely removing the SDK3 package or following the instructions at https://docs.vrchat.com/docs/migrating-from-2018-lts-to-2019-lts#empty-control-panel-or-missing-vrcurlinputfield to reimport the DLLs does not resolve the missing mono script error for the component.
In order to resolve it I have to create a brand new project or revert back to a previous backup before the issue occurred.
Speaking to a number of other world creators about this, they have also seen this happen, and so far am unable to determine what causes or how to fix it.